1. Field of the Invention
The present invention generally relates to a sensor with sound navigation and ranging applications. Specifically, the invention is a passive sensor including a pressure conduction composite sandwiched between two electrically conductive elements so as to sense pressure produced by an acoustic wave via a change in conductance of the composite. Sensors may be arranged to form a new variety of dynamically reconfigurable phased arrays.
2. Description of the Related Art
Sound navigation and ranging, commonly referred to as sonar, is based on the propagation of acoustic signals. An acoustic signal in water is most easily described as a mechanical wave that pushes and pulls water molecules adjacent to the wave as it propagates through water. The result is a localized pressure fluctuation around a mean wherein wave amplitude is the peak pressure reached in one cycle. Wave signals may include simple, single-frequency sinusoidal patterns and complex, multi-frequency patterns.
Sonar systems are categorized as either active or passive. In an active sonar system, an acoustic wave source is part of the sonar system so that acoustic waves are both transmitted and received. Transducers are employed to convert one energy form to another. For example, a hydrophone may convert energy in an acoustic wave to electrical energy and a projector may convert electrical energy to an acoustic wave.
In a passive sonar system, the object of interest is the source of the acoustic wave. As such, a transducer receives acoustic waves and converts energy in the acoustic wave to electrical energy. Unlike active systems, source level is generally not known beforehand and must be gathered via other means.
In sonar applications described in the related arts, multiple transducers are arranged in a dimensionally-fixed, geometric pattern, often referred to as an ‘array’ or ‘phased array’, to capture an incoming acoustic wave.
The general principal behind the use of an array to receive and to transmit acoustic waves is that the arrival phase of the incoming pressure wave is different for each transducer in the array. When signals are combined from all transducers, differently phased signals may add to each other, null each other out, or sum to an intermediate value. Spacing between transducers may be employed to create a ‘beam’ allowing acoustic waves from one or more directions to pass through the transducer array while allowing waves from other directions to be attenuated.
Processing techniques may be employed to enhance the characteristics of the beam. For example, shading is a process by which a transducer signal is either changed in amplitude or electronically phased relative to the signals from the other transducers in the array. Shading allows a beam from the array to be steered in a preferred direction. Transducer arrangement, examples include linear one-dimensional arrays, two-dimensional grids, and three-dimensional cubes and cylinders, further enhances beam properties. It is common for transducers to be spaced at some fraction of the characteristic wavelength of the sonar system.
The related arts include a variety of static arrays with fixed dimensional properties. For example, Benjamin, U.S. Pat. No. 6,671,230, describes a three-dimensional arrangement of electro-strictive polymer elements to transmit and receive in different directions.
Piezoelectric materials and piezoelectric polymer composites are typically employed within transducers for use within sonar systems. Piezoelectricity is the ability of a material to produce an electric charge when subjected to mechanical stress. The effect is reversible so that the piezoelectric material changes shape when subject to an externally applied electric charge. Piezoelectric lead zirconate titanate (PZT) and barium titanate (BT) ceramics and piezoelectric polyvinylidene fluoride (PVDF) and poly(vinylidene fluoride-co-trifluoethylene) (PVDF-TrFE) polymers are several exemplary materials used within transducers comprising sonar systems from the related arts.
Piezoelectric materials and piezoelectric polymer composites are inherently problematic in sonar applications. For example, ferroelectrics are susceptible to the deleterious effects of salt water. As such, ferroelectric materials require special packaging to allow for direct placement in seawater. In another example, the physical nature of piezo-polymers limits the magnitude of the output signal and the signal-to-noise ratio. As such, piezo-polymers require a great deal of amplification and signal conditioning. Also, piezoelectric materials must operate around their resonance for better signal sensitivity thereby limiting bandwidth. Frequency agility may be improved by mechanical or electrical tuning the resonance of the piezoelectric material, as addressed by Newnham et al. in U.S. Pat. No. 5,166,907. However, tuning increases cost and complexity of the transducer.
Therefore, what is currently required is a less expensive, electrically and mechanically simple transducer for sonar applications that minimizes signal amplification and filtering requirements.
Furthermore, what is currently required is an array of non-piezoelectric transducers capable of being reconfigured in real-time without mechanical devices so as to alter the performance characteristics of a sonar system.